1. Field of the Invention
The present invention relates to a process for producing a vinyl chloride homopolymer or a vinyl chloride-based copolymer (hereinafter generically referred to as "vinyl chloride-based polymer"). More specifically, the invention relates to a process by which a vinyl chloride-based polymer of high quality can be produced by suspension polymerization of vinyl chloride monomer or a vinyl chloride-based monomeric mixture (hereinafter generically referred to as "vinyl chloride-based monomeric material") in a polymerization vessel, with high productivity while preventing the formation of polymer scale in the polymerization vessel.
2. Description of the Prior Art
Heretofore, suspension polymerization of a vinyl chloride-based monomeric material has been carried out by a process in which water, the vinyl chloride-based monomeric material, a polymerization initiator, a dispersant and, optionally, other various additives as required, are placed into a polymerization vessel equipped with a jacket, an agitator and a reflux condenser, then hot water is passed through the jacket to heat the reaction system to a predetermined temperature, and thereafter cooling water is passed through the jacket and reflux condenser to remove the heat of polymerization, thereby controlling the temperature of the reaction system to a predetermined temperature.
With the recent trend toward use of larger polymerization vessels and rationalization of production systems, however, various techniques for carrying out polymerization have come to be employed, such as, for example, feeding water at a raised temperature (so-called "hot charge") or automatic feeding of a polymerization initiator by use of a metering high-pressure pump. Consequently, where a polymerization initiator with high activity is used, it is necessary to introduce the initiator into the polymerization vessel after feeding of the monomeric material but before the temperature is raised. In such a situation, the polymerization initiator introduced to the surface of an aqueous mixture containing the monomeric material drifts across a gas-liquid interface portion and the surface of a liquid-phase portion before gradually diffusing into and throughout the bulk of the liquid phase. Depending on the position of introducing the polymerization initiator, therefore, polymer scale deposition in the gas-liquid interface portion can increase. Besides, where an easily foaming dispersant is used, formation of a foamed polymer can occur. Especially where the introducing position of the polymerization initiator is in the vicinity of the wall surface of the polymerization vessel, block form polymer scale can grow on the downstream side with respect to an agitated flow direction, resulting in that a large mass of polymer scale may peel off, to produce a serious trouble such as clogging of an open piping.
Due to the above difficulties, the prior art processes have the drawbacks of an increased idle time, associated with polymer scale removal or the like, and incapability to maintain good polymer quality, because of fish-eye formation or the like.